ouch . Maybe some winter metal damage pics in a few months ? Sounding like some bad ring mojo with 20% and fouled plug

Will find out in the spring:drink:

might have a bad #4 valve seal, leaking oil in and fouling that one cylinder.

starter strength, cam, battery strength, all play a role in compression test #'s.

Can you post pictures of all the plugs?

How many revolutions are you turning the motors over for your compression test?

That's why I posted the Mercruiser Service Bulletin in a past post - per what OEM Mercruiser lists and how they want a compression test done on their marine engines.

1. Is mercruiser wants the engine at room temp not warmed up.

2. They want at least 100 psi in all cylinders,

3. They want any min. compression test # in any one cyclinder should not be less than 70 % of the highest recorded cylinder compression tested #.

4. Also they want 4 compression strokes min to record the final compression #'s.

I was trying to help the OP out with OEM info but I guess not many read it. Here is the SB and it still holds current today as if things would change Mercruiser will send out a revision of the SB like they did in 1999 to replace the 1997 SB to all Merc dealers and class attendees. Link below of the actual Merc SB and they use it for training as well.

www.boatfix.com/merc/bullet/97/97_25.PDF

I tried to do each cylinder five turns new battery, only one was fouled all the rest looked good. But the boat is put away for the winter, tommy if you now what that is:drink:

Sail, that is what I am hoping for, but we will see.

One spark plug fouled - was that fuel fouled or oil fouled ? Also fuel foul and or a lot of excess fuel entering in the cylinder can cause wash down of the cylinder - losing possible ring seal hence lower compression # and high % of leak down test # as air passing by the rings. And of course oil fouled spark plug(s) worn - or scored cylinder walls and or worn rings. Or worn valve guides.

Bup yes I did see what you posted and thank you, the only difference was I did warm the engine a little I did follow the rest.

My compression on the cylinder that fouled was 125-130

And the rest of the #s are ?

All numbers were between 120-130. I also know that my intake had a little water coming out the front, no water I'm the oil.

I also know that my intake had a little water coming out the front, no water I'm the oil.
I'M NOT SURE I UNDERSTAND.

I believe your compression gauge is giving a consistent low reading. I had one last year that I have had for 10 years suddenly do the same as yours when i went to test a motor . Bought a new gauge and the same cylinders were reading 155 instead of the 120 i was getting.

I think you might have a intake leak drawing oil from the valley into that cylinder..

Motors do not wear out to the point that you lose power unless it is a small diesel that had been lugged all its life or if is a motor that runs continuously like on a generator or something like a auto engine being used as a large irrigation pump.And those motors will have 1000s of hours on them before they show signs of wear to the point ayht it can be measured in lost power.

For an example on our parasail boats we were getting 4000 hours in a matter of 3 to 4 years use with no problems out of a 330 hp 454. These motors ran fine when we would replace them just because they had been run in salt and had served their use. They were not lightly used either pulling a parasail is like trying to get your boat up on plane continuously and also a hydraulic pump runs off the front for the winch and that sucks 30 horse off.

so what I am saying is motors do not wear out from normal use unless you abuse one by over revving it or over heating it hard . The stock 454 is very very durable..

I think you are making a mistake pulling your heads. I would pull the intake 1st chance you get and take a look.at that.

It would be worth re-doing the compression test with throttle & choke open as well as engine up to temp. If results are still suspect, then move onto a cylinder leak test, because that will tell you how much leak you have and where it is coming from.

With that info, you can better plan a rebuild or what ever needs to be done.

All of that was done.

Thanks pantera, the boat is put away for the winter I will start early spring and recheck in the spring.

Agreed. Pull the intake and have look. My guess is that you will find oil coming from the intake. Either that or a bad valve seal (which you can see when you remove the intake). I think your long-block is fine.

Thanks I hope it's that easy